Finding a VoIP client that works

For a while I've tried to find an Open Source VoIP client that works. I try SIP Communicator regularly, but it fails to connect to the test numbers of all my different SIP accounts. When I tried QuteCom it seemed to work, so I kept it.

The other day I had to attend a phone conference, so I used QuteCom to dial into the conference  system. I used that system for the first time and somehow couldn't make it let me join the conference. I tried all the codes I had in all the different menus it was offering, to no avail.

After much frustration (which involved a colleague setting up a conference call for me to test with), I got suspicious that the system didn't seem to react to all my key presses. So I checked my VoIP provider's homepage who recommended X-Lite except for Windows 7 which I was using. Instead, it offered a link to PhonerLite, which I installed. And surprise, I was suddenly able to operate the conference system.

I think it's quite surprising that QuteCom seems to generate dial tones that are close to, but not close enough to those required by the standard to be interoperable. What's more, all the VoIP clients mentioned above including SJPhone which I use on OS X have the ugliest user interfaces I had to use in a very long time, ironically in some cases due to lack of styling and in others because of its exuberance. Why isn't there a reliable, easy-to-use open source VoIP client?